A heat exchanger is already known from Japanese Patent Application Laid-open Nos. 59-183296 and 59-63491, which is formed by cutting, into an angle shape having two end edges, opposite ends in a flowing direction of each of first and second heat-transfer plates disposed adjacently and alternately with each other to define high-temperature fluid passages and low-temperature fluid passages. A heat exchanger is also already known from Japanese Patent Application Laid-open No. 58-40116, which includes high-temperature fluid passages and low-temperature fluid passages alternately defined by folding a band-shaped heat-transfer plate in a zigzag fashion.
The volume flow rate of a high-temperature fluid flowing through high-temperature fluid passages in a heat exchanger is not necessarily equal to the volume flow rate of a low-temperature fluid flowing through low-temperature fluid passages in the heat exchanger. For example, in the case of a heat exchanger used in a gas turbine engine, the volume flow rate of a high-temperature fluid comprising a combustion gas is larger than the volume flow rate of a low-temperature fluid comprising air. However, the above known heat exchanger suffers from a problem that the pressure loss of the fluid having the larger volume flow rate is increased and the pressure loss in the entire heat exchanger is also increased, because the lengths of the two end edges of the angle shape are set equal to each other.
When the heat-transfer plates formed in the zigzag folded fashion are disposed radiately to define high-temperature fluid passages and low-temperature fluid passages alternately with each other in a circumferential direction, if an attempt is made to form a heat exchanger having a center angle of 360.degree. from a single folding plate blank, the folding plate blank is required to have a large length, thereby making it difficult to produce the heat exchanger. Moreover, there is a problem that the yield of the blank is degraded. Therefore, it is conceived that a module having a predetermined center angle is formed from a folding plate blank having a suitable length and a plurality of the modules are connected together in the circumferential direction to form a heat exchanger having a center angle of 360.degree.. If the structure of bond zones between adjacent modules are not taken into consideration sufficiently, the following problem is encountered: the heat-transfer plates may be fallen down in the circumferential direction in the vicinity of the bond zones, whereby they may not be arranged correctly in a radial direction, moreover, the heat mass in the bond zones may be increased. Another problem is that if the accuracy of the end edges of the folding plate blank is not controlled precisely, a misalignment is liable to occur between the end edges of the folding plate blank in the bond zones.